Cross-coupling reaction of saccharide-based alkenyl boronic acids with aryl halides: The synthesis of bergenin

Article abstract of DOI:10.1002/chem.201304304

A convenient synthetic pathway enabling D-glucal and D-galactal pinacol boronates to be prepared in good isolated yields was achieved. Both pinacol boronates were tested in a series of cross-coupling reactions under Suzuki-Miyaura cross-coupling conditions to obtain the corresponding aryl, heteroaryl, and alkenyl derivatives in high isolated yields. This methodology was applied to the formal synthesis of the glucopyranoside moiety of papulacandin D and the first total synthesis of bergenin. Building blocks with boron: A convenient synthetic route to D-glucal and D-galactal pinacol boronates was developed, and the boronates were used in cross-coupling reactions to generate the corresponding aryl, heteroaryl, and alkenyl derivatives in high yields (see scheme). This methodology was applied to the formal synthesis of the glucopyranoside moiety of papulacandin D and the total synthesis of bergenin.

Full text of DOI:10.1002/chem.201304304

DOI: 10.1002/chem.201304304
Full Paper
&
CÀC Coupling
Cross-Coupling Reaction of Saccharide-Based Alkenyl Boronic
Acids with Aryl Halides: The Synthesis of Bergenin
Kamil Parkan,*[a] Radek Pohl,[b] and Martin Kotora*[c]
Abstract: A convenient synthetic pathway enabling d-glucal
and d-galactal pinacol boronates to be prepared in good
isolated yields was achieved. Both pinacol boronates were
tested in a series of cross-coupling reactions under Suzuki–
Miyaura cross-coupling conditions to obtain the correspond-
ing aryl, heteroaryl, and alkenyl derivatives in high isolated
yields. This methodology was applied to the formal synthesis
of the glucopyranoside moiety of papulacandin D and the
first total synthesis of bergenin.
Introduction
C-Arylglycosides are an interesting and important class of natu-
ral compounds possessing appealing biological properties that
may have potential application in many areas of medicinal
chemistry. The saccharide moiety of such compounds is usually
composed of d-glucose scaffolds and vitexin, orientin etc.,[1]
with bergenin[2] and papulacandin D[3] serving as typical exam-
ples (Figure 1). However, compounds possessing ribo, manno,
and other scaffolds also exist.[4] Despite the fact that numerous
synthetic procedures based on a range of methodologies have
been described, the development of new and more syntheti-
cally robust procedures is still desirable.[5] This interest is driven
by the weakness of the available methodologies with respect
either to the required reaction conditions or to the substrate
scope.
Figure 1. Selected natural C-glycosides.
One approach to C-arylglycosides is based on the use of the
corresponding 1-alkenylboronic acids or their pinacol esters in
the Suzuki–Miyaura cross-coupling reaction[6] with suitable sub-
stituted aryl and heteroaryl halides, and with alkenyl halides.
During the course of our work on that subject, a report on
a synthetic procedure for the synthesis of C-arylglycosides
based on cross-coupling reaction of glucal boronate with aryl-
(heteroaryl) bromides was published.[7] The report clearly dem-
onstrated the advantages of this approach; however, the
scope of this investigation was limited to just six examples. In
our report we would like to show that the use of pinacol boro-
nates of d-glucal and also d-galactal offers an improved syn-
thetic protocol and extends the scope of the reaction. Finally,
we would like to demonstrate that this methodology can pro-
vide synthetic access to various natural C-aryl glycosides that
have been isolated from a variety of medicinal plants.[8] One
such compound of interest is bergenin,[9] which has interesting
pharmacological properties such as antihepatotoxic, antiulcero-
genic, anti-HIV, antifungal, hepatotoxic, hepatoprotective, anti-
arrhythmic, neuroprotective, anti-inflammatory, and immuno-
modulatory activity. Bergenin-containing extracts from Macar-
anga peltata are used in Indian folk medicine for treatment of
venereal diseases[10] and bergenin itself is an active pharma-
ceutical ingredient of a Chinese drug[11] described to be effec-
tive against coughs and bronchitis. Due to its broad spectrum
[a] Dr. K. Parkan
Department of Chemistry of Natural Compounds
Institute of Chemical Technology
Technickꢀ 5, 166 28 Praha 6 (Czech Republic)
Fax: (+420)220444422
E-mail: parkank@vscht.cz
[b] Dr. R. Pohl
Institute of Organic Chemistry and Biochemistry
Academy of Sciences of the Czech Republic, v. v. i.
Flemingovo 2, 166 10 Prague 6 (Czech Republic)
[c] Prof. Dr. M. Kotora
Department of Organic Chemistry
Faculty of Science, Charles University in Prague
Hlavova 8, 128 43 Praha 2 (Czech Republic)
Fax: (+420)211-951-326
E-mail: kotora@natur.cuni.cz
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201304304.
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of biological activities, the development of new and efficient
synthetic pathway for its preparation is highly desirable. In this
report we would like to show that the approach mentioned
above allows the flexible synthesis of various substituted sac-
charide derivatives and can be applied to the first total synthe-
sis of bergenin.
lowed by hydrolysis. In all cases, the boronic acid 4a was iso-
lated in 75–80% yields, however, although it was obtained
with a reasonable purity (ca. 90%), it was difficult to obtain in
higher quality due to its instability. Attempts to protect the
boronic acid function of 4a with pinacol under published con-
ditions[6] to obtain tri-TIPS glucal pinacol boronate 4b were
not successful and resulted in the formation of complex reac-
tion mixtures. Interestingly, its formation in high yield was
claimed,[7] but its spectral characterization was not reported. In
view of the aforementioned results, we decided to approach
the synthesis of TIPS-protected glucal boronate 4b by other
means (Scheme 1). The procedure was based on a similar ap-
proach, that is, on the reaction of the 1-lithiated glycal 3 with
Results and Discussion
Synthesis of the starting boronic acid derivatives
Our initial attention focused on the synthesis of protected
d-glucal boronic acid and its pinacol esters (Scheme 1 and
Scheme 2). Although boronation of the double bond in d-glu-
cals with B2Pin2 catalyzed by an Ir-complex could be used for
the synthesis of d-glucal pinacol boronates,[12] we were looking
for a more economical method that was also suitable for larger
scale syntheses.
2-isopropoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane
(iPrOB-
pin). The reaction proceeded cleanly, providing the desired
compound 4b in an almost quantitative yield. The successful
isolation strongly depended on appropriate work-up proce-
dure of the formed reaction mixture, and it was essential to
carry out the extraction by using
a mixture of toluene and water
(for details see the Supporting
Information).
In addition to 4b, boronate
4c was also prepared by the re-
action
sequence
described
above (Scheme 2), that is,
d-glucal 1 was converted into
the
bridged-silyl
protected
[24]
glucal 2b with (tBu)2Si(OTf)2
followed by silylation of the free
Scheme 1. Synthesis of d-glucal boronic acid 4a and d-glucal pinacol boronate 4b.
hydroxyl group at position C-3
with TIPS chloride and imidazole.
Compound 2b was then treated
with tBuLi followed by reaction
with iPrOBpin to give the stable bicyclic boronate 4c as a crys-
talline solid, again in almost quantitative yield.
In the next step we wanted to prepare the corresponding
fully protected d-galactal pinacol boronate 7 (Scheme 3). How-
ever, the primary hydroxyl functionality required protection
with a more bulky silyl protective group, because it is known
that the TBS-protected galactal tends to succumb to compet-
ing a-silyl deprotonation at the 6-position.[16,25,26] To avoid this
undesirable competing reaction, we protected the 6-hydroxyl
group of commercially available d-galactal 5 with TIPS chloride
and imidazole followed by in situ installation of the less bulky
TBS groups at the 3- and 4-positions providing the protected
derivative 6 in good yield. An analogous approach based on
C-1 lithiation of galactal 6 with 3.5 equivalents of tBuLi and
Scheme 2. Synthesis of d-glucal pinacol boronate 4c.
Initially, we decided to explore pathways for the preparation
of boronic acid 4a (Scheme 1). Because an alkenyl sugar deriv-
ative is exposed to tBuLi during the course of the reaction,
suitable protective groups that were not affected by strong
bases during the course of reactions had to be chosen for
the saccharide moiety (e.g., methoxymethyl ether
(MOM),[13,14] tert-butyldiphenylsilyl (TBDPS),[13,15,16,17,18]
triisopropylsilyl (TIPS),[13,19,20] di-tert-butylsilylene,[13,21]
and tert-butyldimethylsilyl (TBS)[22,23]. Tri-TIPS-protect-
ed glucal 2, prepared by protection of commercially
available d-glucal 1 with TIPS chloride and imidazole,
was converted into 1-lithiated intermediate 3 by
treatment with 4.0 equivalents of tBuLi. A number of
trialkyl borates B(OR)3 (R=Me, Et, iPr) were added fol- Scheme 3. Synthesis of d-galactal pinacol boronate 7.
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subsequent excess of iPrOBpin was used (Scheme 3). The de-
sired product, the pinacol ester of (1,5-anhydro-2-deoxy-3,4-O-
di-(tert-butyldimethylsilyl)-6-O-triisopropylsilyl-d-lyxo-hex-1-eni-
tolyl)boronic acid (7) was obtained in an almost quantitative
yield. All starting boronic acid derivatives 4a–c and 7 were
fully characterized by means of NMR spectroscopy. In the
13C NMR spectra of these compounds, the signal of C-1 is
broad and usually absent due to 13C–11B spin-spin coupling in-
teraction and additional 10B/11B isotope induced chemical shift
of C-1 signal. This difficulty can be overcome by acquiring H,C-
HMBC spectra for which H-2/C-1, H-5/C-1, and H-3/C-1 cross-
peaks can be observed, providing an assignment of the
C-1 signal. In addition, NMR spectra of galacto-derivatives 7
and 11 measured at ambient temperature provide generally
broad signals, likely due to conformational flexibility. Therefore,
it is advisable to acquire NMR spectra of these derivatives at
higher temperatures (558C in CDCl3, 808C in CD3CN).
Table 1. Cross-coupling reaction of 4a with 8.
Entry
Halide 8
Conditions
Yield of 9 [%][a]
1
2
3
4
5
6
7
8
9
8a
8b
8c
8e
8 f
8g
8i
8j
8k
8l
A
A
A
A
A
A
A
A
A
A
A
89
84
93
72
76
88
70
78
74
74
71
10
11
8m
[a] Isolated yield.
Cross-coupling reactions of 4 with aryl halides 8
Cross-coupling reactions of 4a–c were performed with a range
of aryl, heteroaryl, and alkenyl halides (Figure 2). After exten-
sive screening of various reaction conditions, the best results
manner, providing compounds 9j–l in 78, 74, and 74% isolated
yields, respectively (entries 8–10). Finally, the cross-coupling re-
action with vinyl bromide 8m was performed to give
9m in 71% isolated yield (entry 11). To achieve high
conversions of starting material and high yields of
the corresponding products, the use of 2.1 equiva-
lents of the boronic acid 4a was required. Perhaps
because of this, partial dimerization of 4a to dimer
4d (Figure 3) was observed.[27]
Cross-coupling reactions of 4b were initially also
tested under conditions A. Again, a 1.8-fold excess of
4b was used to ensure good conversions and yields.
Although the cross-coupling reactions with 8b, 8c,
8g, and 8l provided the corresponding products 9b,
9c, 9g, and 9l in high isolated yields (88, 87, 88, and
77%, respectively; Table 2, entries 1, 3, 6, and 10), the
formation of dimer 4d in various amounts was again
observed (5–20%). To suppress the formation of the
latter, conditions B using KOAc as the base were
tested.28 Thus, the reaction of 8b, 8c, 8e, and 8h–j
gave rise to the corresponding products 9b, 9c, 9e,
Figure 2. Aryl, heteroaryl, and alkenyl halides 8 tested in the cross-coupling reactions.
were achieved with [Pd(PPh3)2Cl2] in the presence of 2m aque-
ous solution of Na2CO3 in dimethoxyethane at 808C (condi-
tions A). The results are summarized in Table 1.
and 9h–j in good yields (86, 85, 80, 87, 76, and 80%; entries 2,
4, 5, 7, and 8). Although, the reaction rate was considerably
slower, the formation of dimer 4d was not observed.
Initially, the Suzuki–Miyaura cross-coupling reactions of 4a
with aryl halides were carried out under conditions A (see
Table 1) by using Na2CO3 as the base. The reactions proceeded
well with para-substituted aryl bromides and halides 8a–c,
giving the corresponding products 9a–c in good isolated
yields of 89, 84 and 93%, respectively (entries 1–3). Further-
more, the use of meta- and ortho-substituted aryl halides 8e–g
provided the expected products 9e–g in good yields of 72, 76,
and 88%, respectively (entries 4–6). The reaction with 1,4-di-
bromobenzene 8i gave the double-substituted product 9i in
a good 70% yield (entry 7). The Pd-catalyzed cross-coupling re-
actions with heteroaryl halides 8j–l proceeded in a similar
Cross-coupling reactions of 4c were carried out under condi-
tions A only (Table 3). Thus, reaction with 8c to give 10c pro-
Figure 3. Dimer 4d.
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Table 2. Cross-coupling reaction of 4b with 8.
Table 4. Cross-coupling reaction of 7 with 8.
Entry
Halide 8
Conditions
Yield of 11 [%][a]
Entry
Halide 8
Conditions
Yield of 9 [%][a]
1
2
3
4
8b
8c
8d
8o
A
A
A
A
80
85
79
84
1
2
3
4
5
6
7
8
9
8b
A
B
A
B
B
A
B
B
B
A
88
86
87
85
80
88
87
76
80
77
8c
[a] Isolated yield.
8e
8g
8h
8i
8j
8l
iodopropenoate 8o, providing the product 11 o in 84% isolat-
ed yield (entry 4). Because, to the best of our knowledge, there
have not been any reports on cross-coupling of galactal boro-
nates with organyl halides, these results indicate that this
methodology could also be applied to saccharide pinacol bor-
onates with other scaffolds.
10
[a] Isolated yield.
Table 3. Cross-coupling reaction of 4c with 8.
Synthesis of the papulacandin D glucopyranoside moiety
In the last couple of years several approaches to the papula-
candin D glucopyranoside moiety have been reported. These
methods were based on organolithium addition to gluconolac-
tone,[30,31,32] reductive aromatization of quinols,[33] transforma-
tion of arylvinylfurans,[34] or cross-coupling of a vinylstannane[35]
or a vinylsilanol with aryl halides.[29,36,37] We envisioned that the
papulacandin D glucopyranoside moiety could be also pre-
pared by a cross-coupling reaction of a suitably substituted
aryl halide with boronate 4c. Because we have shown that
cross-coupling between boronate 4c and iodobenzene 8q
(Table 3, entry 3) proceeded successfully to give 10q with high
isolated yield (88%), further conversion of this intermediate
was undertaken (Scheme 4). In the next step, the double bond
was oxidized with dimethyldioxirane (DMDO)[35] under mild
conditions. The epoxidation proceeded stereoselectively to
provide a-epoxide 12, which was immediately opened by in-
tramolecular attack of the pendant benzylic hydroxyl to furnish
the desired spiroketal 13 as a single anomer. The previous syn-
thesis of 13 led to a mixture of a and b-anomers, which had to
be epimerized under acidic conditions[37] to obtain the re-
quired anomer. We assume that the formation of the single
diastereoisomer was caused by the fixed conformation of the
bicyclic derivative that would not cause axial-equatorial flip on
the pyranose ring. Debenzylation of 13 led to the formation of
compound 14 in 93% isolated yield. Subsequent desilylation
followed by peracetylation provided 15 in 89% yield, which is
a known intermediate in synthesis of papulacandin D.[38]
Entry
Halide 8
Conditions
Yield of 10 [%][a]
1
2
3
4
8c
8n
8p
8q
A
A
A
A
87
75
88
87
[a] Isolated yield.
ceeded in good 87% isolated yield (entry 1). This result is com-
parable to results obtained for 4a and 4b. The reaction with
8n proceeded in a similar manner, providing 10n in good
75% isolated yield (entry 2). Next, the reactions with sterically
hindered aryl halides 8p and 8q were attempted (entries 3
and 4). A synthesis of aryl bromide 8p, which is a potential in-
termediate for the bergenin synthesis (see below), has not
been previously reported and had to be synthesized starting
from methyl ester of gallic acid through a reaction sequence
involving acetylation, methylation, benzylation, and bromina-
tion (for synthetic details see the Supporting Information). Aryl
iodide 8q, a known intermediate in the papulacandin synthe-
sis, was synthesized according to a known procedure.[29] Grati-
fyingly, in both cases, the cross-coupling reactions proceeded
smoothly to give the corresponding products 10p and 10q in
very good isolated yields of 88 and 87%, respectively.
Finally, we focused on the Pd-catalyzed cross-coupling reac-
tions of fully protected d-galactal pinacol boronate 7 with sev-
eral aryl halides (Table 4). The reactions of 7 with aryl halides
8b–d proceeded well, with high isolated yields (80, 85, and
79%, respectively) of the corresponding products 11 b–d (en-
tries 1–3). A similar result was also obtained with methyl (E)-3-
Synthesis of bergenin
Prior to the synthetic endeavors that would lead to the synthe-
sis of bergenin, transformations of the cross-coupling products
into stereoselectively pure arylglycosides were attempted.
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drogenation[42] of 8c with H2 on PtO2 also proceeded stereose-
lectively to give aryl-b-C-2-deoxy-glycoside 18 (4.60 ppm (dd,
J1,2 =11.2 Hz, J1,3 2.6 Hz, 1H, H-1)).
In view of the aforementioned results, the synthesis of ber-
genin was executed as follows (Scheme 6). The cross-coupling
product 10p was hydroborated with BH3·Me2S, followed by ox-
idation of the CÀB bond to furnish arylglucoside 19 as a single
Scheme 4. Synthesis of the papulacandin D glycopyranoside moiety 15.
Compound 10c, with the bridged-silyl protecting group to
keep the molecule locked in one conformation, served as
a model (Scheme 5). It was found that hydroboration[39] of the
double bond in 10c with BH3·Me2S complex followed by oxida-
tion with alkaline hydrogen peroxide gave aryl-b-C-glycoside
Scheme 6. Synthesis of bergenin 23.
diastereoisomer. 1H and 13C NMR spectra of 19 measured at
08C in CDCl3 revealed the existence of two isomers. Because
vicinal coupling constants of sugar protons in both isomers
were similar, we concluded that the isomerism is caused by
hindered rotation of the aryl moiety. Unfortunately, NOE meas-
urements did not provide more detailed information on the
orientation of the aryl part for individual isomers. Subsequent
selective oxidation of the benzyl alcohol moiety was accompa-
nied by cyclization to lactol 20, which was further oxidized to
give lactone 21 in 85% isolated yield. The silyl protecting
group was then removed to give 22 (80%) and, finally, reduc-
tive debenzylation provided bergenin 23 in 90% isolated yield.
Thus, bergenin was synthesized in six steps from bromide 8p
in 40% overall yield.
Scheme 5. Transformations of the glucal double bond in 8c and 10c.
Conclusion
16 as a single stereoisomer (4.32 ppm (br. d, J1,2 =9.4 Hz, 1H,
H-1)). The formation of the opposite anomer (5.16 ppm (br.d,
We have developed an efficient method for preparation of
aryl- or alkenyl-C-glucals and -C-galactals by using glucal or
galactal boronates as the key synthons in cross-coupling reac-
tions performed under Suzuki–Miyaura conditions. Partial di-
merization of the boronates observed when K2CO3 was used as
the base could be prevented by the use of KOAc. A possibility
of further functionalization of the double bond in the cross-
coupling products (1-aryl-C-glycals) was explored and resulted
in diastereoselective syntheses of a- or b-glycosides in good
yields. These results enabled the synthetic potential of this
J1,2 =3.8 Hz, 1H, H-1)), aryl-a-C-glycoside 17, was achieved by
epoxidation of the double bond with DMDO[40] followed by
subsequent cleavage of the formed epoxide ring with lithium
triethylborohydride.[41] It should be added that the reactions
mentioned above were also carried out with the tri-TIPS-pro-
tected derivative 8c, but this led to the formation of intracta-
ble reaction mixtures. This is clearly the result of conformation-
al flexibility of the starting material 8c. On the other hand, hy-
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was applied in an alternative synthesis of papulacandin D glu-
copyranoside intermediate 15 in 64% overall yield. Second,
the cross-coupling reaction was utilized as a crucial step for
the first total synthesis of bergenin 23 in 40% overall yield in
only six steps from bromide 8p.
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